Braxton Hicks contractions can start as early as 6 to 12 weeks into pregnancy but are usually not felt until the second or third trimester. Most commonly, women begin to feel Braxton Hicks contractions more noticeably from around the 20th to 28th week, with them becoming more frequent and intense in the third trimester and near the due date.

Key points about timing:

• They may begin anytime after 6 weeks gestation but usually felt from the second trimester onward.

• Often felt starting between 20 to 30 weeks, with increased frequency in the last trimester.

• In some cases, especially if it’s not the first pregnancy, they might be noticed as early as the second trimester.

• Typically, they are most often felt during the third trimester and become more intense as the pregnancy progresses.

Braxton Hicks contractions are irregular, usually last about 30 seconds, and are mostly mild or uncomfortable rather than painful. They are considered "practice" contractions that do not cause cervical dilation or true labor onset.

Thus, Braxton Hicks contractions start generally from early pregnancy but are mostly noticeable from mid-pregnancy onwards, particularly in the second and third trimesters, with increased frequency and intensity closer to delivery.